Patterning of areas in a polymer or resist material using laser ablation is known. The organic layer is, typically, an insulation layer between two wiring planes. Via holes are opened through the organic layer for connecting wires on one level to wires on the other level. U.S. Pat. No. 4,508,749 entitled "Polyimide Films With Ultraviolet Light" to Bannon, et al, assigned to the assignee of the present invention and incorporated herein by reference, teaches a technique for forming patterns in a polymer using laser ablation. Essentially, laser energy, striking the polymer, imparts energy that breaks polymer bonds. The polymer volume expands locally in the area where the bonds are broken. This localized swelling forcibly expels the fragmented polymer from the layer. A mask defines the polymer area to be irradiated.
Laser ablation requires different masks than those used for optical patterning. For non ablation photolithographic (optical) techniques, an opaque pattern is formed on a transparent substrate. Typically, the optical mask is used to form a pattern in photoresist. The photoresist pattern is used to form a pattern in an underlying layer, such as for a wiring plane or an integrated circuit chip layer. However, lasers are seldom employed to expose the photoresist and, when lasers are used, the laser energy is a fraction of that required for laser ablation. However, the material used to form the opaque pattern, e.g., chromium, absorbs laser energy. So, when these optical masks were used for laser ablation, the opaque areas of the optical mask were damaged or destroyed when the organic layer was ablated. Consequently, instead of forming the pattern in the organic layer, the mask is destroyed.
So, for prior art laser ablation masks, the opaque pattern is formed from other materials such as dielectric. U.S. Pat. No. 4,923,772 to Kirch, et al, entitled "High Energy Laser Mask and Method of Making Same" assigned to the assignee of the present invention, and incorporated herein by reference, teaches making a laser ablation mask pattern from multiple dielectric layers. The multiple dielectric layers have alternating high and low indices of refraction that, when overlayed, result in opaque mask areas that exhibit maximum reflectivity of laser energy. As taught by Kirch, et al, making these dielectric masks is a complicated, multistep process. For this reason, dielectric laser ablation masks are expensive.
During mask fabrication, dielectric layer defects may occur that cause mask defects. Also, defects can be inadvertently designed into the mask, by accidentally adding or omitting shapes or by erroneously opening an area, e.g. an extra unwanted via. Defects in a laser ablation mask can be either blocked areas or, areas left open (holes). Since individual dielectric layers do not block the ablation laser, blocked dielectric areas are rare, requiring a repeated defect on several dielectric layers, i.e., enough layers to make an area near opaque. Holes, on the other hand, occur when enough laser energy striking a defective area goes through the mask and ablates the polymer layer, i.e., printing the defect into the polymer layer. These hole defects aren't identified until after the mask is complete. However, once the mask is complete, mask defects, buried in the dielectric layers, cannot be repaired.
Repairing blocked area defects is simpler than repairing holes. Blocked areas may be corrected in the polymer after the pattern is printed, rather than on the mask. Since the defect blocks laser ablation of the polymer, the pattern defect may be corrected by directly ablating the polymer to remove the defect without using a mask.
Unfortunately, omitted shapes, extra vias, or holes may not be repaired so easily. Mask holes, when large enough, print as unwanted holes in the polymer layer, i.e., the laser ablates the polymer at the hole. Omitted shapes, extra vias, or holes in photomasks may be repaired by covering (blocking) the hole. For examples of photomask repair, see U.S. Pat. No. 4,340,654 to Campi wherein an opaque coating material is fused to the defective photomask by an intense source of radiant energy (such as a laser) and see, U.S. Pat. No. 4,463,073 to Miyauchi et al, wherein a metal-organic film on the defective mask is irradiated, rendering the film opaque at the defect. However, because the materials used for these opaque repairs are just as likely to ablate as is the polymer, these repair methods are unusable for permanently repairing laser ablation dielectric masks. Furthermore, holes cannot be refilled in the polymer pattern because, once removed, the polymer cannot be replaced reliably. Consequently, modifying a laser ablation mask to cover an unwanted open, whether to repair a defect or to affect an Engineering Change (EC), meant making a new mask.
Another approach to optical mask repair is used for phase shift masks. Unlike typical optical photomasks, phase shift masks rely on the phase of light striking photoresist from adjacent mask openings. Light passing through a phase shift mask from adjacent openings will either reinforce (in phase) or cancel (out of phase) to form very fine integrated circuit shapes. U.S. Pat. No. 5,085,957, "Method of Repairing a Mask" to Hosono, teaches a repair method for a phase shift mask. In Hosono, an ion beam is directed at a defect to trench the mask surface (.lambda.) sufficiently so that the light passing through the defect is out of phase (shifted) with adjacent pattern light. Thus, the phase shifted light from the defect is cancelled by the adjacent light and the defect is not printed.
However, phase shifting is not applicable to laser ablation for a number of reasons. First, the transmission openings which are narrow enough to pass x-rays, would appear opaque to the ablation laser. Second, a laser is not diffuse enough that, regardless of phase, energy from one transmissive area would interfere with another adjacent transmissive area. Thus, shifting the laser's phase would not affect the defect. So, for the reasons photomasks make poor laser ablation masks, prior art methods of photomask repair are not practical to repair holes in a laser ablation dielectric mask.
This defect problem is compounded by the fact that larger images are being ablated to improve manufacturing efficiency. A larger image requires a larger mask. Mask defect rates are a measure of the number of mask defects per unit area. Typical laser ablation mask defect rates are 0.000083/mm.sup.2. For a 15 mm by 15 mm mask, approximately 1 mask in 100 will have a defect. When only 1 in 100 masks is bad, remaking a defective mask is a minor annoyance. However, for a large laser ablation mask, 150 mm.times.150 mm, the probability of a defect rises to 100%. Thus it has become impossible to make defect free large laser ablation masks. Therefore, defective laser ablation masks must be repairable.